Alkalinity

Alkalinity
Definition
 Measure of of water’s capacity to neutralize
acids=Acid neutralizing capacity
 Without this acid-neutralizing capacity, any acid
added to a stream would cause an immediate change
in the pH.
 Alkalinity in natural water is due to:
 Salts of weak acids
 Carbonate, bicarbonate
 Borate, silicate, phosphate
 A few organic acids resistant to biological oxidation (humic
subs.)
 In polluted or anaerobic watersAcetic, propionic acid, H2S
 Weak or strong bases
 Ammonia
 Hydroxides
Types of alkalinity in natural waters :
1. Hydroxide
2. Carbonate
3. Bicarbonate
 For most practical conditions, alkalinity due to other
materials in natrual waters is insignificant and can be
ignored
Public Health Significance
 The alkalinity of water has little public health
significance.
 High alkaline waters are usually unpalatable
Alkalinity Measurement
 Titration with N/50 H2SO4
 Reported in terms of mg/L CaCO3
 specifies that the sample has an alkalinity equal to that
of a solution with a certain amount of calcium carbonate
(CaCO3) dissolved in water.
 If sample pH >8.3 titration is done in two steps
 1. Titration until pH=8.3 (till phenolphtalein end pointpink to colorless-)
 2. Titration until pH=4.5 (till bromcresol end point)
At pH=8.3
At pH=4.5
A simple example
0.01 M [HCO-3 ]
10 meq/L * 50 mg/meq = 500 mg/L
Methods of expressing alkalinity
Phenolphtalein and Total Alkalinity
@ pH 10 all the hydroxide ions are neutralized
@ pH 8.3 carbonate converted to bicarbonate
Titration till phenolphthalein end point
Phenolphthalein alkalinity
Total alkalinity  titration till pH 4.5
Conversion till carbonic acid H2CO3
Hydroxide, Carbonate and
Bicarbonate Alaklinity
1. Calculation from alkalinity
measurements
2. Calculation from alkalinity and pH
measurement
3. Calculation from equilibrium equations
(carbonic acid)
1. Calculation from alkalinity
measurements
 Based on assumptions and total/phenolphtalein
alkalinity measurements
 Assumption: Hydroxide and bicarbonate alkalinities
cannot be present at the same time (incorrect but
rough estimate)
Five possible situations :
1. Hydroxide only
2. Carbonate only
3. Hydroxide and Carbonate
4. CO=3 and HCO-3
5. HCO-3
@pH 8.3 neutralization of hydroxides are completed.
Hydroxide alkalinity
pH usually >10
Titration is essentially complete at pp end point
Hydroxide alkalinity = Phenolphtalein alkalinity
Carbonate only
pH usually >8.5 or higher
Titration is pp end point is exactly
equal to one-half of the total titration
Carbonate alkalinity = Total alkalinity
Hydroxide-carbonate
pH usually >10 or above
Titration from pp to bromcresol
end point represents one-half of
the carbonate alkalinity
Carbonate-bicarbonate
pH usually >8.3 and <11
Titration to pp end point
represents one-half of
the carbonate alkalinity
Bicarbonate
pH < 8.3
Bicarbonate
alkalinity= total
alkalinity
2. Calculation from alkalinity + pH
measurements
 Should measure
 pH
 Total alkalinity
 Phenolphtalein alkalinity
 Calculate hydroxide, carbonate, bicarbonate alkalinity
2. Calculation from alkalinity + pH
measurements
 First calculate OH alkalinity from pH measurement
2. Calculation from alkalinity + pH
measurements
 Second, make use of the principles of the first
procedure to calculate carbonate and bicarbonate
alkalinity
 Titration from pH 8.3 to 4.5 measures the remaining one
half of the carbonate + bicarbonate.
 or
Alkalinity and acidity are based on the “carbonate system “ .
[Alk.]=[HCO-3 ] + 2[CO=3 ] + [OH-] – [H+ ]
( mol/L of H+ that can be neutralized)
(Alk.)=(HCO-3 )+ (CO=3 ) + (OH-) – (H+)
( eq/L of H+ that can be neutralized)
Alk. In mg/L as CaCO3 = ( Alk.) x EW CaCO3
Example :
CO=3 = 20 g/m3
OH- = 0.17 g/m3
HCO-3 = 488 g/m3
Alk. = ?
Ion
MW
( g/mole)
EW (g/eq)
(eq/m3 )
CO=3
HCO-3
OH-
60
61
17
30
61
17
20/30=0.67
488/61=8
0.17/17=0.01
[H+ ] [ OH- ] = Kw
(OH-) (H+) =Kw
[H+ ] = 10-14 / (0,01x 1/1000 x 1 mol/eq)
= 10-9 mol/L =10-9 eq/L = 10-6 eq/m3
[Alk.]=[HCO-3 ] + 2[CO=3 ] + [OH-] – [H+ ]
(Alk.)= 8,00 + 0,67 + 0,01 - 10-6
=8,68 eq/m3
(8,68 x 10-3 eq/L) x (50000 mg/eq)=434 mg/L as CaCO3
Expressing in terms of CaCO3
Species A
mg/L as CaCO3 = ( mg/L A)(EW CaCO3 / EWA)
Example : 10 mg/L Mg2+
Mg+2 = 24,3 mg/L
EW Mg+2 = 24,3/2=12,15
Conc of Mg+2 as CaCO3
(10 mg/L)x((5000 mg/eq)/(12150 mg/eq Mg+2))
= 41,15 mg/L as CaCO3
3. Calculation from equilibrium
reactions
Application of alkalinity data
 Chemical coagulation: excess alkalinity should be
present
 Water softening: important in calculating lime and
soda ash requirements
 Biological processes
 Industrial wastewaters: Many municipalities prohibit
caustic alkalinity to sewers
 İSKİ requires 6<pH<12
pH changes during aeration of water
 Common purpose of aeration is to strip
 Carbondioxide  pH
 Ammonia
 VOCs
 Air content 0,035 % by volume CO2
 Henry ‘s constant : 1500 mg/L.atm
Equilibrium conc.for CO2 = 0,00035 x 1500 =0,45 mg/L
KA1 = [H+ ] [HCO-3 ] / [H2CO3 ]
If alkalinity = 100 mg/L
Aerated until equilibrium of CO2 in air pH=8,6
pH changes in the presence of algal
blooms
 Algae use CO2 in photosynthesis.
 Algae can reduce CO2 conc. below its equilibrium
concentrations.
 During algal blooms pH 10 can be seen
 Algae can continue to extract CO2 until inhibitory pH
(10-11)
 As pH increase alkalinity forms change
 Total alkalinity remains constant unless CaCO3
precipitation occurs
Boiler waters
 Carbondioxide is insoluble in boiling water and
removed with steam.  pH  shift of alkalinity
from bicarbonate to carbonate, and carbonate to
hydroxide CaCO3 precipitate

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